In general, a vuilleumier heat pump device used in such as an air conditioner comprises a hot-side heat pump and a cold-side heat pump, as disclosed in Japanese Patent Application laying Open Gazette No. 1-187164. In the vuilleumier heat pump device, each of the heat pumps is so composed that a displacer is housed in a cylinder. Both the displacers of the hot-side and cold-side heat pumps are connected to each other so as to reciprocate at a set phase angle (e.g., 90.degree.). A hot space and a middle-temperature space in the hot side cylinder are communicated with each other through a hot side passage, while a cold space and a middle-temperature space in the cold side cylinder are communicated with each other through a cold side passage.
Reciprocations of both the displacers change volumes of respective spaces above-mentioned in the cylinders, so that a working gas is changed in pressure to form a thermodynamic cycle. Heat input is conducted at a heater and a cooler on the hot side and cold side passages, while heat radiation is conducted at middle-temperature heat exchangers on both the passages. Further, in the hot-side heat pump, heat is stored in a regenerator on the hot side passage when a working gas moves from the hot space to the middle-temperature space, while heat stored in the regenerator is regenerated when a working gas moves from the middle-temperature space to the hot space. In the cold-side heat pump, heat is stored in a regenerator on the cold side passage when a working gas moves from the middle-temperature space to the cold space, while heat stored in the regenerator is regenerated when a working gas moves from the cold space to the middle-temperate space.
In the above-mentioned vuilleumier heat pump device, a rod of one displacer is connected to a rod of another displacer via a connection mechanism. A crank shaft of the connection mechanism is connected to an auxiliary driving motor as an auxiliary driving power source. The auxiliary driving motor enables the displacers to reciprocate. That is, when a rotation speed of the crank shaft increases over its self operating point, the auxiliary driving motor gives auxiliary driving power to the crank shaft. On the contrary, when the rotation speed of the crank shaft decreases below its self operating point, the auxiliary driving motor gives a reverse load to the crank shaft.
In detail, as shown in FIG. 4, a shaft power (shaft work) Wt of the crank shaft increases linearly as the rotation speed N of the crank shaft increases. In contrast to this, a power loss L which is the sum of a mechanical loss and a flow loss increases in a curve as the rotation speed N of the crank shaft increases. Both the variation characteristics of the shaft power Wt and the power loss L are intersected at a set rotation speed N. The point of intersection is the self operating point S.
At the self operating point S, the shaft power Wt is balanced with the power loss L so that power supply to the auxiliary driving motor is interrupted. When the rotation speed N increases over the self operating point S, the power loss L exceeds the shaft power Wt so that the auxiliary driving power is required (See A1 in FIG. 4). As a result, the auxiliary driving motor is activated.
On the contrary, when the rotation speed N decreases below the self operating point S, the power loss L is below the shaft power Wt so that a reverse load is required (See A2 in FIG. 4). As a result, the auxiliary driving motor is reduced in speed.
There is known a conventional vuilleumier heat pump device in which the crank shaft as above-mentioned is connected to a motor and a brake, as disclosed in Japanese Patent Application Laying Open Gazette No. 2-4174. In this prior art, a required rotation speed of the crank shaft which corresponds to a required load is calculated, e.g., during a cooling operation, there is calculated a required rotation speed of the crank shaft which corresponds to a required heat flow in the cooler on the cold side passage. Then, cooling capability is controlled in such a manner that when the required rotation speed exceeds the rotation speed at the self operating point S, the motor is driven, and that when the required rotation speed is lower than the rotation speed at the self operating point S, the brake is operated.
Further, there is known another conventional vuilleumier heat pump device in which the crank shaft as above-mentioned is connected to a brake, as disclosed in Japanese Patent Application Laying Open Gazette No. 4-198671. Also in this prior art, operation capability is controlled in such a manner that when a cooling load or a heating load decreases, the brake is operated to decrease a rotation speed of the crank shaft so that the number of reciprocations of the displacer per unit time is decreased.
In the above prior arts, however, the self operating point S is fixed. Therefore, when the rotation speed N changes, i.e., when the reciprocation speed of the displacer changes so that the rotation speed N increases over the self operating point S, the auxiliary driving motor must be operated, thereby lowering operation efficiency.
On the contrary, when the rotation speed N decreases below the self operating point S, surplus power is generated so that the heat flow to be inputted from the heater increases. Accordingly, an indicated COP (coefficient of performance) (indicated COP=Wc/We, We: a working gas work in a hot space, Wc: a working gas work in a cold space) is lowered and a reverse load is generated, thereby increasing variations of the rotation speed.
In order to adjust the shaft power Wt, there is a method in which the temperature of the hot space is changed by changing the temperature of the heater. In this method, however, an operation with a set efficiency cannot be performed.
In view of the foregoing problems, the present invention has its object of enhancing the COP, reducing variations of the rotation speed and performing an operation with a set efficiency.